The present invention relates to electrical power distribution systems, and more particularly, to methods and apparatus for contactless transfer (especially magnetic transfer) of electric power from primary electric conductors to secondary pick-up coils.
In many applications, passenger and cargo transport systems such as trains or monorails carry electric rotating or linear motors to provide propulsion. The motors for such systems generally have brushes for proper distribution of the electric energy within the motors. The electric power is produced by power supplies. In addition, the power supplies for these transport systems usually use either on-board batteries or pantographs that draw electric power to the transport system from conductors that parallel the route of the transport systems. The electric power can also be supplied by means of busbars with sliding contact-type current collectors, flexible-cable festoon systems, or cable reels, as well as other cable handling devices.
Many applications impose extraordinarily strenuous operating conditions. These include the need for higher speed and/or acceleration, complex track configurations, and difficult environmental conditions.
Battery life limits the utility of battery-powered transport systems. Sparking, noise and high installation costs limit the utility of pantographs and/or the motors. Wear and tear and maintenance costs limit the utility of all of the passenger transport systems described above because they are unreliable and maintenance intensive.
The difficult environmental conditions make conventional transport systems vulnerable to water, wind, snow and ice, as well as explosive atmosphere, dirt and other possible ambient situations. In addition, conventional transport systems can be hazardous in operation, producing, for example, arcing and sparking, as well as being electrically charged and, therefore, not touch-proof.
Contactless inductive power transfer offers an attractive alternative to the transport systems described above because it is free of sparking, wear and tear and hazardous operation. Such power transfer is also safe, quiet and marked by a high reliability. Further, contactless inductive power transfer offers unlimited speed and acceleration. Prior art proposals of contactless inductive power transfer systems have not resulted in a wide usage of contactless power transfer because satisfactory inductive transfer of electric power can only be accomplished by taking additional factors into account.
In the prior art, a number of patents have issued to disclosing inductive electric power transfer to moving devices. Generally all of these prior art patents describe the transfer of small quantities of electric power since a relatively high quantity of apparent power is required as a consequence of the large air gap in such prior art systems.
There have also been a number of patents describing motive energy transfer (for example, Tesla, in U.S. Pat. No. 514,972). However, the historic patent that is the most relevant to the present invention is that of Hutin, et al. (U.S. Pat. No. 527,857) which, in 1984, described the use of alternating current induction at approximately 3 kHz. In 1974, Otto (in New Zealand Patent Number 167,422) suggested a practical solution for inductive power transfer using a series resonant secondary winding operating in the range of 4 to 10 kHz for the inductive power transfer to a moving vehicle.
In 1994 Boys and Green (U.S. Pat. No. 5,293,308) suggested another practical system for one-way inductive power transfer, using the results of Otto with regard to the resonant secondary winding and adding some devices to improve the transfer characteristics. The Boys-Green system adds a capacitor in parallel to the primary. This method reduces the required apparent power but has at least two disadvantages. One disadvantage is that the point of compensation varies with the secondary load. The power factor of this and other prior art systems is load-dependent and never equals unity. The other disadvantage of the Boys-Green system is that a large amount of reactive power circulates in the primary, resulting in high primary losses and lower efficiencies which are unfortunately nearly independent of the transferred power. To reduce the effects of these disadvantages, Boys, et al. suggest tuning a primary parallel capacitor at a ringing frequency that depends on, and is disturbed by, the secondary load conditions. Consequently, only limited amounts of real power can be transferred in these prior art systems, leading to their marginal utility. Boys, et al. also suggest using Litz cable for the primary in order to reduce the losses in the primary. Further suggestions reflect the need for special design of control and hardware components to achieve other and less no important power transfer characteristics. For example, complex primary-secondary magnetic decoupling is required for multiple secondaries, and complex primary segmenting and tuning design results in system constraints.
In 1993 Nishino and Boys (New Zealand patent application NZ93/00032) suggested forming the primary from a number of modules that are pre-tuned primary segments connected in series. Linking poles with the same polarity with a non-inductive cable tends to constrain their system, limiting the possible resonant frequencies.
The present invention provides an improved system for the inductive (magnetic) transfer of large quantities of electric power due to its high efficiency, simple design and low installation costs. It accomplishes this, in part, through its novel pickup coil design, unique power factor compensation and power transfer circuitry, and reverse power flow capability. The invention is applicable to systems which include AC or DC sources and one or more AC and/or DC, active and/or passive secondary loads. The simplified design of the invention accommodates the use of standard components and thereby reduces the installed cost for typical applications.
The inventive contactless power system (CPS) overcomes the following limitations, among others: it provides forward and reverse power transfer capability; it has a unity power factor under all load conditions; it applies only real power to the primary, leading to higher efficiency and greater power transfer capability; the quantity of transferred power is limited only by the primary capacity; primary-secondary magnetic decoupling is not required for multiple secondaries; and it has a simple primary geometry without any system constraints.
The inventive system has a large number of aspects. It is a universal contactless power system which magnetically transfers large quantities of in-phase (i.e., power factor=1) electrical power bidirectionally between an AC or DC primary source and one or more AC and/or DC secondary loads which are active and/or passive.
The inventive system has a distributed-winding pickup coil that improves primary-secondary magnetic coupling, which increases efficiency and permits greater power transfer. The pickup coil has parallel compensation, and consists of either fixed resonant parallel capacitors or parallel capacitors with additional adaptive compensation. This additional adaptive compensation transfers in-phase power to the load at a constant voltage, regardless of the magnetic or state (i.e., consuming or generating) of the load. In contrast to the prior art and as an additional aspect, the present invention uses a new pickup coil design that consists of two windings which are partly magnetically coupled and partly magnetically not coupled. The two windings are each distributed on the middle yoke and a distinct one of the side yokes of a ferromagnetic core, leading to significantly improved magnetic coupling and power transfer efficiency between the primary inductive loop and the pickup coil.
In another aspect of this invention, the two windings of the pickup coil are each connected to a parallel resonant capacitor and compensated to unity power factor.
In another aspect of the invention, the parallel capacitors partly compensate the windings and additional components adaptively compensate the collective windings to unity power factor and automatically supply in-phase power to the secondary load at a constant voltage regardless of the magnitude of the load.
The inventive system also offers series compensation of the primary loop, which results in a constant unity power factor under all load conditions and increases efficiency and permits greater power transfer. This compensation is accomplished through distributed series capacitors, or with concentrated transformer-coupled capacitors.
In contrast to the prior art and as an additional aspect of this invention, the primary inductive loop is compensated to unity power factor with one or more series capacitors and consequently the present invention does not require any reactive power circulating in the primary loop; rather, the power applied to the primary and magnetically transferred to the secondary is always at unity power factor.
The reverse power control provided by the inventive system allows reverse power from an active load to be transferred back through the pickup coil to the primary and then fed back into the line or to other intermediate consumers.
The pickup coils of the inventive system can be connected in parallel at their DC output for greater power transfer.
The inventive system can also provide a primary constant current control, which allows power transfer to multiple secondaries without the need to magnetically decouple the secondaries from the primary. This can be accomplished by pulse control of the primary output inverter.
The inventive system can further provide primary constant voltage control, which allows current to rise and fall with the magnitude of the secondary load, and to fall to the magnetization level when the load is zero. This increases efficiency and permits greater power transfer.
In addition, the inventive system can have parallel primary conductors, which increases the ampere-turns of the primary inductive loop and thereby provides greater power transfer. This can be accomplished through current balancing, which minimized losses and increases efficiency.
The inventive system can also have a three-phase primary inductive loop and a secondary pickup coil for greater power transfer. This can be accomplished through a three-phase primary output inverter and a three-phase secondary input inverter.
The inventive system can also offer a branch secondary configuration, in which applications involving additional motion axes can be powered from a single primary system.
Additionally, the inventive system can provide multiple primary zones, which are switchable between magnetically active and magnetically neutral. This allows power transfer only when a zone is magnetically active and, consequently, increases system safety and efficiency. Zone control can allow only one load per zone and consequently places all secondaries in parallel so that all secondaries have a common constant voltage source and additionally ensures that any one secondary cannot physically be in contact with any other secondary. This is an anti-collision system.
The inventive system can also provide multiple primary output inverters with load-share switching to multiple primary inductive loops, parallel primary inverter bridges for greater power transfer, and a primary inductive loop made of non-Litz standard industrial cable.
In one aspect, and in its most general form, the invention is a contactless system to magnetically transfer in-phase electric power between a primary source and a secondary load. The system includes a primary energy converter, a primary energy transfer network which is magnetically coupled to a secondary energy transfer network, and a secondary energy converter.
In accordance with one aspect, the invention is a contactless system to magnetically transfer electric power from an input power source to a secondary load. the system includes a primary energy converter, a primary inductive loop, a secondary pickup coil, and a secondary energy converter. The primary energy converter is connectable to the input power source and includes an output inverter. The primary inductive loop is connected to the output inverter and includes at least one turn which is compensated to unity power factor. The secondary pickup coil is magnetically coupled to the primary inductive loop and compensated to unity power factor. The secondary energy converter is connected to the secondary pickup coil, includes an input inverter, and is connectable to the secondary load.
In another aspect, the invention is a contactless magnetic system to transfer in-phase electric power from a primary source to multiple secondary loads.
In another aspect, the invention is a contactless magnetic system to transfer in-phase electric power bidirectionally between a primary source and one or more active secondary loads.
In another aspect of the invention, one or more identical pickup coils are connected in parallel at their respective DC outputs to increase the total output power.
In further aspects of the invention, the series compensation of the primary inductive loop is accomplished either by distributed series capacitors or by concentrated transformer-coupled capacitors. The latter method eliminates multiple compensation locations and eases compensation adjustment.
In contrast to the prior art and as another aspect of this invention, the secondary power controller includes reverse power control which senses the state of the active load and controls the flow of reverse power when the load is in a generating state. The reverse power is transferred back to the primary where it is fed back into the line or, alternatively, to other intermediate consumers.
In contrast to the prior art and as a further aspect of this invention, the primary power controller includes constant current control which enables power transfer to multiple secondary loads without the need to decouple the secondary pickup coils from the primary inductive loop.
In another aspect of this invention, the primary constant current control is accomplished via variable pulse control of the primary output inverter.
In contrast to the prior art and as another aspect of this invention, power is supplied to the primary inductive loop at a constant voltage and unity power factor. The magnitude of the primary current is determined by the magnitude of the load and drops to the magnetization level when the load is zero.
As another aspect of the invention, the primary inductive loop is comprised of multiple parallel turns which are current-balanced. The ampere-turns of the primary and thus the power transfer is increased.
As additional aspects of the invention, the primary inductive loop and secondary pickup coil magnetically couple three-phase power and the primary output inverter and secondary input inverter are three-phase bridges.
In yet another aspect of the invention, the secondary energy converter feeds an auxiliary inductive loop which is coupled to an auxiliary pickup coil and energy converter. This arrangement allows power transfer to equipment operating in multiple axes such as the bridge and trolley of an overhead crane.
In another aspect of the invention, the primary inductive loop is configured into multiple zones which can be switched between magnetically active and magnetically neutral such that power can be magnetically transferred to a secondary only when the zone is magnetically active. In yet another aspect of the invention, control means is included to permit only one load to be located in any one zone, thereby placing all loads in parallel and preventing the physical collision of one load with another.
In another aspect of the invention, multiple primary energy converters are connected to multiple primary inductive loops in a manner which allows one primary energy converter to feed more than one primary loop through a switching arrangement. By this configuration a primary energy converter may be taken out of service without disrupting power transfer to the overall system.
In another aspect of the invention, the primary energy converter utilizes parallel output inverter bridges to increase the primary power for greater power transfer.
In another aspect of the invention, the primary inductive loop is made of conventional industrial cable (non-Litz) which is made possible by the high efficiency of the invention.
The contactless transfer of electric power disclosed in the application occurs over a large air gap whose separation is measured in the range of centimeters. The physical principle of the disclosed invention is based on Maxwell""s laws as they relate to alternating magnetic fields.
Although only one or a few specific applications of this invention will be disclosed in the application, the fields of application generally involve the transfer of power to moving or parked equipment such as commercial or industrial vehicles, cranes, elevators, material handling systems, machine tools, and other similar equipment.
In accordance with one aspect, the invention is a contactless inductive system to transfer electric power to a first load. The system includes a first pick-up coil, a primary, a first AC-inverter, and a constant current controlled chopper. The first pick-up coil is tuned at a ringing frequency. The primary is formed as a loop. It is also connected with one or more capacitors in series, and tuned at the ringing frequency.
The first AC-inverter is also tuned at the ringing frequency. The first AC-inverter feeds the primary system with constant voltage or constant current and maintains its power factor equal to one, independent of the first load.
The constant current controlled chopper feeds the AC-inverter, so that the electric power transferred to the first load has a unity power factor.
In accordance with a further aspect, the invention is a contactless inductive system to transfer electric power from a primary system to a first load. The contactless inductive system includes a first pick-up coil, a primary, a first AC-inverter, and a constant current power supply. The first pick-up coil includes two windings which are partly magnetically coupled and partly magnetically decoupled. Each of the two windings are connected with a resonant capacitor in parallel, so that the pick-up coil is tuned at a ringing frequency.
The primary is formed as a loop and connected with one or more capacitors in series which are tuned at the ringing frequency.
The first AC-inverter is tuned at the ringing frequency, and feeds the primary system with constant voltage or constant current. The first AC-inverter also maintains its power factor equal to one, independent of the first load. The constant current power supply feeds the AC-inverter with electric power.
In accordance with yet another aspect, the invention is a contactless inductive system to transfer electric power between a first system and a second system. The first system is alternatively operable as a source of electric power and a consumer of electric power. The contactless inductive system includes a first coil and a second coil. The first and second coils are tuned at a ringing frequency.
The system also includes a first control circuit connected between the first coil and the first system, and a second control circuit connected between the second coil and the second system. The first and second control circuits are operable 1) to supply electric power to the second system when the first system is operable as a source of electric power and 2) to supply electric power to the first system when the first system is operable as a consumer of electric power.
In contrast to the prior art, the present invention uses a new pick-up coil design that consists of two windings which are partly magnetically coupled and partly magnetically decoupled, where each winding is connected to a resonant capacitor. In accordance with this invention, the two sides of the primary have a self-symmetry because they carry identical currents, but also allow different voltages along the pick-up coil. The total output power is generated by two full bridge rectifiers, each of which is assigned to a distinct one of the windings of the pick-up coil. In further accordance with the present invention, it is also possible to increase the DC-output power without connecting the two pick-up coil windings directly in parallel. (Connecting the two pick-up coil windings directly in parallel would lead to an unacceptable increase of pick-up losses due to different stray magnetic fields from the two windings of the pick-up coil.) Attempting to increase the DC-output power by using a larger cross-section cable has the disadvantage of increased eddy current losses and geometric limitations.
In a further aspect of the invention, the DC-output of one or more identical pick-up coils can be connected in parallel to increase the output power.
In yet another aspect of the invention, distributing each of the two windings of an inventive pick-up coil both on the middle yoke and a distinct one of the two side yokes leads to an increase of the magnetic coupling between the primary coil and the pick-up coil. This, in turn, means an increase of the efficiency of the coupling.
The primary coil of the system is fed with a constant current, thereby decoupling the various secondary loads which generally constitute a transport system. As a new aspect of the invention, a constant AC-current in the primary coil is generated by a current control circuit which is connected to a high frequency AC-output inverter. This arrangement keeps the power factor of the primary coil always at one regardless of the load, and leads to a minimum required voltage and minimal AC-output inverter installation costs. Additionally, the required current in the primary is minimized relative to the known prior art. Therefore, the eddy current losses in the primary are minimized so that no fine stranded Litz cable is required, but rather standard industrial Litz cable can be used.
The contactless transfer of electric power disclosed in the present application occurs over a large air gap whose separation is measured in the range of centimeters. The physical principle of the disclosed invention is based on Maxwell""s laws as they relate to alternating magnetic fields. Although only one or a few specific applications of this invention will be disclosed in this application, the fields of application are generally moving or rotating power consumers, such as vehicles, cranes, elevators, material handling systems, and machinery tools.
According to still another aspect, the invention comprises:
one or more pick-up coils that are 1) assigned to one or more secondary capacitors connected in parallel with the windings of the pick-up coils, 2) tuned to the ringing frequency of the coil, and 3) connected to one or more bridge rectifiers that are connected in parallel at the DC-output;
a buck converter which is assigned to each secondary power consumer and used to keep the output voltage constant in case of different loads on the secondary;
a primary cable which is formed as a loop and which includes one or more turns;
one or more series capacitors which are connected in series to the primary and tuned at the ringing frequency;
a high frequency AC-inverter which feeds the primary system with a constant voltage or a constant current; and
a constant current controller which is required as a decoupling device in the case of multiple secondary power consumers.
The separation of the pick-up coil winding into two individual isolated windings which are partly magnetically coupled on a middle yoke of an E-shaped iron core and partly magnetically decoupled on the side yokes of the core leads to a number of advantages. Asymmetric effects due to individual stray fields do not lead to additional losses since the symmetry is self-adjusted because of the individual magnetic coupling of each pick-up coil winding with one side of the primary.